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1. Field of Invention
This invention pertains to the magnetic tape recording, and particularly to the control of tape speed and tape write and read operations.
2. Related Art and Other Considerations
In a magnetic tape drive, information is recordable on magnetic tape in the form of magnetic flux transitions. The magnetic flux transitions are typically created by a write element mounted on a head. As relative motion occurs between the head and the tape, a track of information is recorded on the tape. In a read mode, magnetic flux transitions recorded on tape are sensed by a read head as the read head follows a track on the tape.
Typically, but not always, the relative motion is motion of the tape past the head(s). Whatever one or more of the tape and the head is in motion, the motion is usually motor driven. For example, in some tape drives, known as cartridge drives, tape extends between supply and take-up reels housed in a cartridge which is insertable into the drive. Typically the tape is transported in the cartridge as a result of activation of a motor. The motor may either cause rotation of a tape-contacting capstan, a tape-contacting belt, or one or more of the reels.
In many tape drives, the speed of the motor (whether the motor moves the head or moves the tape) is controlled by constant rate clock, usually driven by a crystal oscillator. In other systems, in a read mode the motor speed is controlled by information read from the tape, e.g., from a servo signal pre-recorded on the tape.
In operation, the tape drive is connected to a host. In a record mode, the host sends to the drive, and particularly to a buffer of the drive, data which is to be recorded on the tape. From the buffer the data is transmitted through a write channel to the write head(s) for recording on the tape. In a read or reproduction mode, when the host requests data from the tape, the tape is read and data transduced therefrom is transmitted to the host via a read channel and the buffer. The tape reading and recording operations are conducted at constant rates, e.g., in accordance with a fixed clock signal generated by a crystal oscillator.
The host has its data rates for transferring data to or from a tape drive. Efforts are usually made to obtain compatibility of the data rates of the host compatible and the tape reading and recording operations of the drive by matching the devices or independently adjusting transfer parameters in one or more of the devices. But the very nature of tape drive operation precludes perfect compatibility. In fact, the buffer is necessitated by differences in the actual data rates of the host and of the drive. For example, certain physical operations of the drive, such as a change of track, create situations in which the data rate of the host cannot maintain the supply of or requirement for data by the drive. The typical resolution to data rate mismatch is insuring that the buffer is of a sufficiently large size to accommodate the data rate incongruity in envisioned circumstances. However, the price of buffer memory is expensive, and the buffering of large amounts of data increases complexity of operation of the drive.
What is needed therefore, and an object of the present invention, is a dynamic linking of the data rate of a host and the data rate of a magnetic tape drive.